Abstract
ElectroMagnetic (EM) reasons resulting in temperature dependence of L-band Vegetation Optical Depth (L-VOD) are currently overlooked in remote sensing products. Discrepancies in retrievals of geophysical surface properties over vegetated areas can result from this incompleteness. This perception motivated to explore EM considerations in how temperature drives L-VOD of a boreal forest. Thereto, a novel physics-based model is developed and evaluated to assess L-VOD sensitivities to canopy temperature and some other model parameters. The L-VOD model is compared to L-VOD derived from close-range L-band brightness temperatures measured through the tree canopy at the Finnish Meteorological Institute's Arctic Research Center (FMI-ARC) in Sodankylä (Finland) during a 4-week and a 1-day period in 2019. Furthermore, the model's ability to explain L-VOD retrieved from brightness temperatures of the “Soil Moisture and Ocean Salinity” (SMOS) satellite over the “Sodankylä grid cell” is investigated. Experimental L-VOD are maximal at around 0 °C and decrease when canopy temperature is moving away from zero degree Celsius. This temperature response, observed at different temporal- and spatial scales, is captured by the proposed L-VOD model and explained by freezing tree sap-water and the dependence of water permittivity on temperature. The demonstrated EM-induced temperature dependence suggest caution with interpreting satellite-based L-VOD, because increased L-VOD around the freezing point is not solely due to increased biomass or rehydration of the vegetation. Further, our study can find future application to compensate L-VOD for EM-induced temperature sensitivity. This potentially leads to improved explanatory power of temperature normalized L-VOD for characterization of forest phenology. Furthermore, we suggest examining the presence and strength of the demonstrated L-VOD temperature response as a practical L-VOD retrieval quality assessment method under steady forest phenology.
Highlights
Temperate, tropical- and boreal forests cover more than 30 % of the Earth’s land surface
L-band Vegetation Optical Depth (L-Vegetation Optical Depth (VOD)) derived from below-canopy ELBARA-II brightness temperatures
Diurnal minima and maxima of TTree are used as the baseline information to decide on the time-period most relevant to our research questions about the EM rea sons of temperature dependence of L-VOD τC: i) What is the change in LVOD derived from close-range and Soil Moisture and Ocean Salinity” (SMOS) L-band brightness tempera tures when the forest canopy goes through transitional freeze/thaw states? ii) Is it possible to explain respective experimental temperature responses τC(TC) by the developed L-VOD model using meaningful values for the model parameters?
Summary
Temperate-, tropical- and boreal forests cover more than 30 % of the Earth’s land surface Their areal extent and phenology affects the ex change of radiative energy, water (via evapotranspiration and infiltra tion/runoff), and trace gases (water vapor and CO2) between the ground and the atmosphere (Richardson et al, 2013). There are indications that earlier spring snow-melt has increased the overall carbon uptake of boreal forests (Pulliainen et al, 2017). It is still disputed if the resulting net carbon flux of this second largest biome is enhanced or dampened through various feedbacks (Magnani et al, 2007). Remotely sensed forest phenology becomes increasingly important to further constrict modeled climate scenarios
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